1. Technical Field
The invention relates to tapered roller bearings with the ability to handle thrust loads in both axial directions.
2. Background Information
Roller bearings for handling thrust loads in both axial directions are well known in the art and are sometimes referred to as bi-directional tapered roller thrust bearings. One application for such bearings is as a main shaft bearing for a gas turbine engine. Gas turbine engine major or xe2x80x9cprimaryxe2x80x9d bearing thrust loads are in one of either the forward or rearward directions, but there are circumstances where the bearings must deal with significant, but much smaller xe2x80x9cminorxe2x80x9d bearing thrust loads in the opposite or reverse direction from the primary loads. (Hereinafter, xe2x80x9creversexe2x80x9d thrust loads means thrust loads that are in the opposite direction of the primary thrust loads.) A bi-directional tapered roller bearing is able to handle both the major and minor thrust loads, thereby eliminating the need for a costly, energy consuming thrust balance mechanism, or eliminating the need for a second, smaller thrust bearing for accommodating the minor reverse thrust loads.
One such bi-direction roller thrust bearing is shown and described U.S. Pat. No. 5,735,612 xe2x80x9cSingle Row Tapered Roller Bearingxe2x80x9d by G. P. Fox and J. R. Dietric, incorporated herein by reference. In its basic form the bi-directional tapered roller thrust bearing has an inner annular tapered race on its cone and an outer annular tapered race on its cup, with tapered roller elements disposed circumferentially in the space therebetween for rolling contact between the roller elements tapered surfaces and the inner and outer races. At the large diameter end of the cup race a cup rib face extends radially inwardly adjacent the large diameter ends of the roller elements; and at the small diameter end of the cone race a cone rib face extends radially outwardly adjacent the small diameter ends of the roller elements. During primary thrust operation, the large diameter ends of the roller elements press against the cup rib face; but most of the major thrust loads are transferred from the cone into the cup (and thence into the structure supporting the cup) through the roller elements via the line contact between the roller element tapered rolling surfaces and the races. This is due to the fact that under primary thrust loads the roller elements are xe2x80x9cwedgedxe2x80x9d between the races.
On the other hand, during reverse thrust loading, the roller elements are substantially unloaded from contact with the races. The cone rib face presses against the small diameter ends of the roller elements, and the cup rib face presses against the large diameter ends of the roller elements, thereby transferring the reverse thrust load through the roller elements from the cone rib to the cup rib in a direction generally perpendicular to the end faces of the roller elements.
The drawback of prior art bi-directional tapered roller bearings occurs during primary thrust loading, not reverse thrust loading. When experiencing primary thrust loads, the contact forces between the bearing races and the tapered rolling surfaces of the roller elements have turned out to be too large for some applications, such as where space is at a premium, and low weight coupled with long life are required. One such application is gas turbine aircraft engines. For example, if a prior art bi-directional tapered roller bearing having the requisite life expectancy is too large for the available space, then it cannot be used. Or, if the space is available, the weight penalty may be unacceptable. In either case, the benefits of the bi-directional tapered roller bearing would not be available for that application. It is desired to have a bi-directional tapered roller bearing that does not have the aforementioned drawback.
The present invention is a bi-directional tapered roller bearing having a first cone rib ring thrust face (face xe2x80x9cCxe2x80x9d, for future reference) adjacent and facing the small diameter end faces of the roller elements, a second cone rib ring thrust face (face xe2x80x9cBxe2x80x9d, for future reference) adjacent and facing the large diameter end faces of the roller elements, and a cup rib ring thrust face (face xe2x80x9cAxe2x80x9d, for future reference) adjacent and facing the large diameter end faces of the roller elements, the bearing components being constructed and designed such that during primary thrust load operation there is a gap between thrust face C and the small diameter end faces of the roller elements and a gap between thrust face A and the large diameter end faces of the roller elements, and at the same time there is contact between thrust face B and the large diameter end faces of the roller elements.
In the prior art bi-directional tapered roller bearing configuration of U.S. Pat. No. 5,735,612, cited above, there are only thrust faces corresponding to thrust faces A and C. There is no thrust face B. As discussed in the Background Information section of this specification, in the prior art bearing the primary thrust loads were forcing the large diameter end faces of the roller elements against thrust face A. In accordance with the present invention, by adding a thrust rib ring (with its thrust face B) to the bearing at the large diameter end of the cone race, and by maintaining a gap between thrust face A and the large diameter end faces of the roller elements during primary thrust operation, the primary thrust loads now force the roller element end faces against thrust face B. The result is that bearings of the present invention will have significantly lower contact forces between the roller elements and races, as compared to the race contact forces on the same size bi-directional tapered roller bearings of the prior art. The reason for the lower contact force on the races is that the centrifugal forces generated by the roller elements during primary thrust load operation add to the contact forces against the cup race of the prior art design, but subtract from the contact forces against the cone race of the present invention.
In applications where high contact forces between the roller elements and races of a prior art bi-directional bearing necessitate the use of a bearing that is undesirably large in order to achieve a certain desired life expectancy, the present invention allows the use of a smaller bearing. Alternatively, the substitution of a prior art bi-directional bearing with a bearing of the present invention of the same size results in a longer bearing life. As stated earlier, the bearing of the present invention is particularly useful in aircraft gas turbine engines, where weight is critical and space is limited. For one particular gas turbine engine application it was estimated that the present invention could increase the bearing life by a factor of four or more compared to the same size bi-directional tapered roller bearing design of the prior art.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.